1. Field of the Invention
The invention relates to an audio playing system, and more particularly to an audio playing system preventing electromagnetic interference (EMI).
2. Description of the Related Art
Currently, audio signal amplifiers are implemented mainly by Class D amplifiers. A Class AB amplifier, a formerly implemented amplifier, generates a linear signal, and a Class D amplifier generates a pulse-width modulation (PWM) signal which comprises audio frequency signals, a PWM switch signal, and PWM harmonic waves. When a Class D amplifier operates, the metal-oxide semiconductor field-effect transistor (MOSFET) impedance during an output stage switches to low from high, and the MOSFET operates in the active region for a few seconds. Thus, power consumption for an output stage of a Class D amplifier is much lower than that of a Class AB amplifier. Moreover, an inductor plus capacitor (LC) filter within the Class D amplifier saves enough power during each operating period of the Class D amplifier, so that power consumption when the Class D amplifier is turned on or off does not occur in speakers connected to the Class D amplifier.
However, electromagnetic interference (EMI) is a problem for a Class D amplifier. Because operations, such as turning on and off transistors, are continuously performed in a Class D amplifier, EMI may occur more easily. When a Class D amplifier is used to drive an earphone, connection lines of the earphone exacerbate EMI. Thus, improving the problem of EMI for a Class D amplifier and enhancing quality of audio played by an audio signal amplifier are important issues.